Sulfur-bearing carbonaceous fuels when burned for purposes of producing energy also produce pollutants such as oxides of sulfur, especially sulfur dioxide. Sulfur dioxide in the atmosphere undergoes photo-oxidation to form sulfur trioxide which, under certain conditions, is converted to sulfuric acid or a sulfate aerosol. An excess of these pollutants is detrimental to the health of mankind, the growth of plants, and in cases of severe excess the useful life of various materials of construction and articles of manufacture. Reduction in the sulfur content of solid carbonaceous materials prior to their conversion into energy offers a solution to the reduction of sulfur pollutants in the atmosphere.
Sulfur exists in solid carbonaceous materials as either organic sulfur or inorganic sulfur. Inorganic sulfur comprises pyritic sulfur, sulfide sulfur, sulfate sulfur, and in some instances other forms of inorganic sulfur generally in relatively small amounts. Organic sulfur is the sulfur which forms a part of organic molecules of the carbonaceous material; pyritic sulfur comprises iron pyrite, FeS.sub.2 ; and sulfide sulfur comprises, for example, FeS and CaS. "Pyritic sulfur" as used herein does not comprehend "sulfide sulfur" as used herein. Sulfur bound to organic molecules is often considered the most difficult form of sulfur to essentially completely remove because of the strong complex organic bonds which tightly hold the sulfur.
U.S. Pat. No. 2,878,163 discloses a process for desulfurizing petroleum coke by mixing the petroleum coke after it has been pulverized with a solid alkali metal hydroxide selected from the group consisting of sodium hydroxide, potassium hydroxide, and lithium hydroxide, heating and maintaining the mixture at a temperature of at least about 700.degree. F. to react with the impurities in the petroleum coke.
U.S. Pat. No. 3,387,941 discloses a process for desulfurizing carbonaceous material by treating with steam in an intimate mixture containing such material and an alkali metal hydroxide, oxide, carbide, carbonate, or hydride at a temperature of about 500.degree. to 850.degree. C. while the hydroxide of the alkali metal is a liquid.
U.S. Pat. No. 3,393,978 discloses a process for de-ashing carbonaceous material by treatment with a mixture containing an alkali metal compound and the carbonaceous material with steam at a temperature above the melting point of the hydroxide of the alkali metal, washing the treated material with water, and treating the water-washed material with an aqueous solution of a water-soluble inorganic acid capable of forming water-soluble salts of the impurities.
Molten media have also been used to desulfurize carbonaceous material, such as U.S. Pat. No. 4,087,514 to Robinson et al employing a basic molten bath of an alkali metal or alkaline earth metal salt of a hydroxyaryl or an alcohol, and in U.S. Pat. No. 3,919,118 to Robinson et al employing a molten bath of an alkaline salt from an organic acid comprising the cresylic acid fraction from the distillation of coal tar and a caustic material selected from the group consisting of alkali metal, and alkaline metal oxides, hydroxides, carbonates, acetates or mixtures thereof.
U.S. Pat. No. 3,812,017 discloses a process for desulfurizing coal by mixing crushed coal with an acid selected from phosphoric acid, phosphorous acids and mixtures thereof, removing the excess acids, if any, from the mixture, and heating the mixture to a temperature between about 400.degree. and about 1100.degree. C. for at least about 15 minutes. The process is said to remove a significant quantity of the organic sulfur and substantially all of the inorganic sulfur.
U.S. Pat. No. 3,824,084 discloses a process for desulfurizing a coal containing pyritic sulfur by treatment with water and air at an elevated temperature and pressure to convert the pyritic sulfur to water-soluble ferrous and ferric sulfate. The resulting slurry is then filtered to produce a coal product of diminished sulfur content.
Carbonaceous material may also be desulfurized by treatment with acid followed by hydrogen treatment as in U.S. Pat. No. 4,054,421 to Robinson et al.
U.S. Pat. No. 3,402,998 discloses a process for desulfurizing a fuel which is undergoing gasification, cracking, hydrogasification, or hydrocracking by use of a hydrogen sulfide acceptor particle such as calcine dolomite.
U.S. Pat. No. 3,640,016 discloses a method for desulfurizing caking coal without substantial caking, devolatilization and conversion to coke or char by mixing a solid particulate getter material with the particulate sulfur-containing caking coal, passing hydrogen gas through the mixture to form a fluidized bed of the particulate mixture, which is maintained at a temperature of 600.degree. to 800.degree. F., to form a nongaseous sulfide by reaction of the coal with the getter and the hydrogen, and separating the nongaseous sulfide and unreacted getter material from the mixture to recover uncaked particulate desulfurized coal without substantial devolatilization. It is further disclosed that various thermodynamic studies of the reaction: EQU FeS+H.sub.2 .fwdarw.Fe+H.sub.2 S
indicate that even at temperatures as high as 1300.degree. F., the equilibrium partial pressure ratio of hydrogen sulfide to hydrogen is still so low that a tremendous recycle of hydrogen is necessary. At equilibrium conversion some 400,000 s.c.f. of hydrogen have to be recirculated to reduce the sulfur in one ton of coal from 4 to 0.5 percent.
U.S. Pat. No. 2,983,673 discloses a process for desulfurizing petroleum coke by first fracturing the coke particles and then treatment with hydrogen or steam or other agents such as sulfur dioxide.
It is known that sulfur may be removed from fluid coke by treatment with hydrogen or hydrogen-containing gases at a temperature of 2400.degree. F. by the formation of hydrogen sulfide, as in U.S. Pat. No. 2,872,384.
U.S. Pat. No. 3,251,751 discloses a process for desulfurizing char particles as they descend by gravity with a counter-current stream of reducing gas, such as hydrogen, which removes a portion of the sulfur in the char as hydrogen sulfide. The gas should be substantially free of hydrogen sulfide.
U.S. Pat. No. 3,009,781 discloses a process for the reduction of the sulfur content of coke particles by treatment with hydrogen sulfide in a fluidized bed heated by the passage of electricity through the bed to produce carbon disulfide.
U.S. Pat. No. 2,721,169 discloses a process for desulfurizing a high-sulfur petroleum coke from a fluidized coking process by subjecting the coke particles to low-temperature oxidation with an oxygen-containing gas followed by a hydrogen treatment. The process involves subjecting the coke particles in the form of a dense turbulent fluidized bed to a low-temperature oxidation treatment followed by a hydrogen treatment. The hydrogen treatment is conducted at a temperature of about 1200.degree. to 1700.degree. F., preferably at 1400.degree. to 1500.degree. F., at atmospheric pressure or above. The coke is contacted at suitable temperature and pressure with a flow of hydrogen which sweeps the hydrogen sulfide product out of the hydrogen treatment zone. The minimum flow of hydrogen utilized is about 100 volumes of hydrogen per volume of coke per hour. Treatment time ranges from 10 minutes to about 10 hours.
U.S. Pat. No. 3,909,212 discloses a process for desulfurizing solid carbonaceous fuels such as coal or lignite by reacting the fuel with oxygen and steam so as to generate nascent hydrogen at the surface and within the fuel particle for reaction with the sulfur in the fuel to form hydrogen sulfide. The reaction is conducted at a pressure of at least about 2 atmospheres and at a temperature of about 1100.degree. to about 1500.degree. F. A portion of the fuel is burned to reach the elevated temperature required.
The effect of air, steam and hydrogen on the desulfurization of bituminous coal, together with an oxidative pretreatment followed by hydrogen treatment, is discussed in a publication entitled "Fuel," Volume 54, at page 113 (1975). A coal, pretreated by heating while exposed to air, was shown to desulfurize faster than the coal which was not pretreated.
U.S. Pat. No. 3,214,346 discloses a process for removing ash components from fluid coke by increasing the porosity of the fluid coke particles by low-temperature oxidation and/or steaming followed by either treatment with gaseous reagents such as chlorine and hydrogen chloride, or by extraction with reagents such as aqueous alkaline materials, acids, and the like. For example, after the low-temperature oxidation of the high surface area coke particles, they may be extracted with an ammonium hydroxide solution or with a sodium hydroxide solution.
Sulfur bearing coal char may be desulfurized as in U.S. Pat. No. 4,053,285 to Robinson et al by treatment at elevated temperatures with carbon dioxide and steam.